1. Field of the Invention
The present invention relates to an information processing device, a receiving method, and a wireless communication system.
2. Description of the Related Art
Non-contact communication is a wireless technology that performs data transmission over distances ranging from zero to several centimeters and is used, for example, in an RFID system that is configured from a non-contact IC card and a reader/writer. The communication can be divided into two types, communication from the reader/writer to the card and communication from the card to the reader/writer, according to the communication direction. In this specification, the former is called downlinking, and the latter is called uplinking. In both of the communication directions, the reader/writer constantly generates a carrier frequency, and the card performs transmission processing and receiving processing based on electric power that it obtains from the carrier frequency.
Non-contact communication methods include an electrostatic coupling method, an electromagnetic induction method, a radio communication method, and the like. Of these methods, the electromagnetic induction method performs data communication by magnetic coupling between a primary coil in the reader/writer and a secondary coil in the card (that is, the two coils operate as an LC resonance circuit). The reader/writer performs the downlink data transmission by performing amplitude modulation of a magnetic field, that is, the carrier, that is generated by the primary coil, and the carrier is detected by the card. In contrast, load modulation is used for the uplink, and the load resistance of the secondary coil is switched based on transmission information in the card. In the reader/writer, the electromagnetically coupled input impedance varies as the load of the electromagnetically coupled secondary coil varies, causing the output level of the carrier frequency to vary. It is therefore possible for transmission information to be read by the card by looking at the changes in the output level.
For example, ISO/IEC IS 18092 (NFC IP-1), which became an international standard in December, 2003, is the non-contact communication standard that governs the specifications of the reader/writer. This standard is the successor to the standard originally used for the Sony Felica (registered trademark) and the Philips Mifare, which became widely used as non-contact IC cards. In the Felica format, Manchester encoding is used, and both the downlink and the uplink use the same packet structure. The packet structure for the Felica format is shown in FIG. 17. The packet that is shown is configured in three parts: preamble, sync, and data. The preamble is a sequence of zeroes six bytes long, and the sync is a known two-byte sequence, “0xB24D”. The data part includes a one-byte LEN that indicates the packet length, the data proper (the payload) in a number of bytes equal to (LEN-1), and a two-byte cyclic redundancy check (CRC) code. The three parts are all Manchester encoded.
In this case, for example, the Manchester encoding changes from a low level to a high level in the middle of the bit period when a binary “0” is transmitted (changing input “0” to “01”) and, conversely, changes from a high level to a low level in the middle of the bit period when a binary “1” is transmitted (changing input “1” to “10”). In other words, it is a coding format that divides a single bit period into a front cell and a rear cell, expressing a logical value “0” when the front cell is at a low level and the rear cell is at a high level, and expressing a logical value “1” when the front cell is at a high level and the rear cell is at a low level. It can also be said that the Manchester encoding converts a single input bit into two bits (or transmits a single bit in two pulses (2 T)), with the communication rate being halved by doubling the bandwidth, but the direct current component being eliminated from the transmitted signal.
The six bytes of zeroes in the preamble portion are Manchester encoded. Therefore, “01” is transmitted forty-eight times in a sequential waveform. The “0xB24D” in the sync portion is also Manchester encoded. In the data portion, the transmitted information, the length information (LEN), and the CRC are Manchester encoded as a group.
On the packet recognize side, a clock (sampling timing) is extracted based on the preamble portion, which is a sequential waveform. In this specification, this operation is called timing synchronization. Next, the sync portion, whose pattern is the Manchester encoded “0xB24D”, is detected, and the starting position of the data portion that follows it is extrapolated. In this specification, this operation is called frame synchronization. Next, the data portion is decoded based on the starting position.
Any number of proposals have been made for a receiving circuit that decodes the Manchester code into a non-return-to-zero (NRZ) code (for example, refer to Japanese Patent Application Publication No. JP-A-11-146022, Japanese Patent Application Publication No. JP-A-11-251916, and Japanese Patent Application Publication No. JP-A-2005-160042).
Incidentally, the communication rates that are prescribed for the Felica format are all multiples of 212 kbps, such as 424 kbps, 848 kbps, 1.7 Mbps, 3.4 Mbps, and the like. As the communication rate increases, the frequency bandwidth of the transmission signal becomes wider in proportion to the communication rate. As the frequency bandwidth of the signal becomes wider, the effects of the frequency characteristics of the transmission route, the transmission RF analog circuit, and the receiving RF analog circuit increase. Generally, the frequency characteristics attenuate more as the frequency becomes higher. The phase characteristics also become more irregular as the frequency becomes higher. This means that the received waveform becomes more irregular as the communication rate for the signal increases.
One method for compensating for the irregularity of the received signal in high-speed communication and the like is adaptive equalization processing (for example, refer to Japanese Patent Application Publication No. JP-A-2004-64681, Japanese Patent Application Publication No. JP-A-2008-22422, and Japanese Patent Application Publication No. JP-A-2008-27270). An adaptive equalization circuit may be configured from a finite impulse response (FIR) filter and a learning circuit. FIG. 18 is an explanatory figure that schematically shows a configuration of a FIR filter. The FIR filter is provided with a delay line in which a plurality of delay elements are connected in series. The FIR filter is able to produce an equalized signal by taking a time series of data inputs, the number of which is equal to the number of the arrayed delay elements, using a multiplier to perform weighting of the data inputs with a tap coefficient that corresponds to the filter characteristics, then performing processing that averages the cumulative total of the data inputs. A known learning signal is transmitted from the transmission side to the receiving side. Ordinarily, a random pattern is used for the learning signal. The learning circuit on the receiving side adjusts the tap coefficient of the filter such that when an irregular learning signal is received through the transmission route, the equalized signal that is output from the FIR filter comes close to the desired signal.
In the performing of the adaptive equalization, a random pattern must be transmitted that is of sufficient length for the tap coefficient of the FIR filter to be learned. At the same time, in order for the data portion within the packet to be decoded from the start, it is necessary for the learning of the FIR filter to be completed at a prior stage.
A method that inserts a random pattern of sufficient length for learning between the sync portion and the data portion, a method that transmits a special packet for learning before the ordinary packet, and the like are conceivable as ways to complete the learning of the FIR filter at a stage prior to the arrival of the data portion. However, the implementing of these methods has the potential to create a problem with interchangeability, because the packet format is used that is different from the Felica format that is prescribed by the NFC IP-1 standard. Furthermore, the time for transmitting information is reduced in order to make time to transmit the random pattern for learning, which is a known signal, thus leading to a drop in the communication rate.
Accordingly, a method for performing adaptive equalization has been proposed that uses the existing packet format of the Felica format in order to improve the adaptive equalization performance. Specifically, the method uses the sync portion of the packet, which is a known signal sequence, for the adaptive equalization learning on the receiving side. However, in the packet format of the Felica format, the number of Manchester encoded bits in the sync portion is at most 32 bits, so the number of learning cycles is not really adequate.
For the method for performing adaptive equalization that uses the existing packet format of the Felica format, a method has been proposed that, for example, performs the learning of the front half of the sync portion at high speed and performs the learning of the rear half of the sync portion at low speed, in order to implement the adaptive equalization learning with little convergence error at high speed.
However, in order to produce favorable learning results by the start of the data portion, it is necessary to restrict the number of the FIR filter taps to several taps. On the other hand, if there are few FIR filter taps, it is difficult to describe the frequency characteristics with precision, so a problem arises, because in some cases, depending on the effects of the frequency characteristics that are received through the transmission route, sufficiently good receiving characteristics will not be achieved, even if adaptive equalization is performed.
For example, in a case where the communication rate is 3.4 Mbps, the Manchester encoded channel rate is 6.8 Mbps, and the bandwidth for the baseband signal is 6.8 MHz. Furthermore, the shortest wave (1 T) in the Manchester encoded signal is 3.4 MHz, and the longest wave (2 T) is 1.7 Mhz, so the signal spectrum is 3.4 MHz, distributed around a center of 1.7 MHz. In this case, when the number of the taps of the FIR filter is five, it is possible to describe five frequency positions within the 6.8 MHz baseband bandwidth. However, only one frequency position that can be controlled exists between 1 T and 2 T. Therefore, in a case where the frequency characteristics of the transmission route between 1 T and 2 T are complex, it is difficult to describe the inverse characteristics of the frequency characteristics and difficult to perform the equalization well.
Thus, in a non-contact communication system that utilizes electromagnetic coupling, in order to make the system compatible with a faster transmission rate while conforming to the Felica format, the adaptive equalization must improve more complex frequency characteristics with a low number of FIR filter taps.
Accordingly, a method has also been proposed that improves the receiving performance, while using the same number of FIR filter taps, by computing the bias of the received signal, performing adaptive equalization of a half sampled signal, and performing a binary determination.